This invention relates to the art of chemical pulping and bleaching of wood pulp, particularly the art of removal, by chemical means, of residual lignin remaining in wood pulp produced by chemical pulping processes, more specifically use of chlorine to remove residual lignin from or bleach wood pulp produced by sulfate or "kraft" pulping processes.
The conventional classical multistage chlorine and chlorine dioxide based bleaching sequence for kraft pulp is one wherein the unbleached kraft pulp is treated with chlorine, conveniently in the presence of a small quantity of a viscosity preserving agent, such as chlorine dioxide, then extracted with aqueous alkali at high pH values, conveniently greater than pH 10, followed by treatment with either hypoclorite followed by chlorine dioxide or chlorine dioxide alone, a second strong caustic extraction and a second chlorine dioxide treatment. These sequences are conventionally represented as C.sub.D EHDED and C.sub.D EDED wherein C represents the treatment with chlorine; H represents treatment with hypochlorite; D represents treatment with chlorine dioxide and E represents extraction at pH values greater than 10 with aqueous alkali. Thus, for example, C.sub.D represents treatment with chlorine in the presence of chlorine dioxide.
Extraction of the chlorinated lignin in the first alkaline extraction stage in the above sequence results in substantial darkening of the pulp compared to the color of pulp at the end of the chlorination stage and a substantial amount of the chemicals employed in subsequent stages are necessary simply to remove the color added by the alkaline extraction.
The use of caustic soda as the alkali in the extraction stages of multistage chlorine based bleaching has long been traditional because it was efficient in removing the reaction products of lignin and chlorine or chlorine dioxide and because it was readily available and relatively inexpensive, being the coproduct of the manufacture of chlorine from brine. The industrial use of chlorine has been declining in recent years and, therefore, the price of caustic soda has risen while the supply has declined to the point where occasional shortages have occurred. Efforts to find alternatives for caustic soda in the bleaching field have, to the inventors knowledge, been limited to the investigation of alternative alkalis capable of providing aqueous solutions of high pH values. Illustrative of these studies is one by Liebergott et. al. reported in Reports in the 69th Annual Meeting, Technical Section, Canadian Pulp and Paper Association, Montreal, 1982, p. A41. which examines a number of alkalis as potential substitutes for caustic soda in the chlorine bleaching process and which reaffirms that caustic soda is the best, that alternative alkalis can be employed and that all work better when the pH values are higher.
The present invention provides alternative extraction methods and reagents, principally in the first extraction step, for chlorinated lignin which minimize darkening of pulp during extraction or permit use of less chemicals to reach brightness levels comparable to those attainable with the conventional treatment, or which permit the attainment of higher brightness levels with equivalent chemical usage than those produced by conventional treatment. The invention also surprisingly shows that the particular aqueous extractants contemplated are effective at pH values lower than those taught as necessary by the prior art.